1. Field of the Invention
This invention relates to an image formation apparatus for finally forming a toner image on paper under an image formation condition that can be controlled, and a toner amount measurement apparatus and a toner amount measurement method for measuring a toner amount.
2. Background of the Invention
Hitherto, an image formation apparatus such as printers, copiers, and facsimile machines adopting electrophotography has been known. In such an image formation apparatus, light is applied to the surface of a photosensitive body for forming an electrostatic latent image and toner is deposited on the electrostatic toner image for development and then the toner deposited on the electrostatic toner image on the surface of the photosensitive body is transferred onto paper by means of a transfer device, a transfer belt, etc., whereby a toner image is finally formed on the paper. In such an image formation apparatus, to form a high-quality toner image, the amount of toner deposited on the photosensitive body or the transfer belt is measured with a toner amount measurement apparatus and the image formation condition applied for forming a toner image is controlled in response to the measured toner amount. An optical measurement method is widely known as a measurement method of the amount of toner deposited on the photosensitive body.
Here, the principle of a toner amount measurement method in a general toner amount measurement apparatus will be discussed with reference to FIGS. 1 to 4.
The surface of a photosensitive body or a transfer belt on which toner is deposited generally has a mirror structure high in flatness; hitherto, such a surface characteristic has been used to measure the toner amount in the toner amount measurement apparatus. Hereinafter, photosensitive bodies, transfer belts, etc., for supporting toner will be collectively called toner supports.
FIG. 1 is a drawing to show the measurement principle of a toner amount measurement method using specular reflection.
In the toner amount measurement method using specular reflection, light L1 of a predetermined strength is applied from a light source 2 such as a light emitting diode to the surface of a toner support 1 and is specularly reflected on the surface of the toner support 1 and reflected light L2 is received by a photosensor 3 such as a photodiode, which then outputs a voltage responsive to the strength of the received reflected light L2.
The reflected light L2 is blocked in the portion of the surface of the toner support 1 where toner is deposited, and as the reflected light L2 is blocked, the light reception amount of the photosensor 3 is lowered accordingly and the output voltage is lowered.
FIG. 2 is a graph to show the relationship between the toner deposition amount and the output voltage of the photosensor in the toner amount measurement method using specular reflection.
The graph shows on the horizontal axis the amount of toner deposited on the surface of the toner support and on the vertical axis the output voltage of the photosensor. The output voltage of the photosensor corresponds to the light amount of the specularly reflected light on the surface of the toner support, as described above.
As a curve 5 inclined downward to the right in the graph shows, the output voltage of the photosensor is lowered with an increase in the toner deposition amount. Since such a curve is previously found, the amount of toner deposited on the surface of the toner support can be found based on the relationship indicated by the curve 5 and the output voltage of the photosensor.
By the way, as for color toner, if light is applied to color toner, scattered light occurs because of reflection on the surface and in the inside of the color toner. A toner amount measurement method using such scattered light is also known.
FIG. 3 is a drawing to show the measurement principle of the toner amount measurement method using scattered light.
Also in the toner amount measurement method using scattered light, light L1 of a predetermined strength is applied from a light source 2 to the surface of a toner support 1 in a similar manner to that in FIG. 1; in the toner amount measurement method using scattered light, however, a photosensor 6 is provided at a position at a distance from the reflected light L2 shown in FIG. 1 and scattered light L3 caused by toner 4 deposited on the surface of the toner support 1 is received by the photosensor 6, which then outputs a voltage responsive to the strength of the received scattered light L3.
FIG. 4 is a graph to show the relationship between the toner deposition amount and the output voltage of the photosensor in the toner amount measurement method using scattered light.
Like the graph of FIG. 2, the graph of FIG. 4 shows the amount of toner on the horizontal axis and the output voltage of the photosensor on the vertical axis. The output voltage of the photosensor corresponds to the light amount of the scattered light caused by the toner.
As a curve 7 in the graph of FIG. 4 shows, the output voltage of the photosensor is raised with an increase in the toner deposition amount. Since such a curve 7 is previously found, the amount of toner deposited on the surface of the toner support can be found based on the relationship indicated by the curve 7 and the output voltage of the photosensor.
Most image formation apparatus in related arts measure the toner amount using either of the measurement principles shown in FIGS. 1 and 3 or measure the toner amount using both the measurement principles in combination.
By the way, with the toner amount measurement method using specular reflection, the measurement sensitivity is degraded if the surface of the photosensitive body or the transfer belt is completely covered with toner.
FIG. 5 is a graph to show the measurement sensitivity in the toner amount measurement method using specular reflection.
The graph shows the toner amount on a toner support on the horizontal axis and the light amount of specularly reflected light on the vertical axis. The inclination of the graph represents the measurement sensitivity.
As the toner amount increases, the inclination of the graph is lessened and in the toner amount exceeding 0.5 mg/cm2, the inclination of the graph is extremely small. Thus, when the toner amount exceeds 0.5 mg/cm2, if the toner amount changes, the light amount of the specularly reflected light scarcely changes and it is very difficult to measure the toner amount. However, the toner amount to be actually measured may extend to 0.5 mg/cm2 or more on the photosensitive body, in which case the toner amount measurement method using specular reflection is not adequate.
On the other hand, with the toner amount measurement method using scattered light, the toner amount at up to 0.7 mg/cm2 level can be measured. However, the toner amount measurement method using scattered light involves some problems. The first problem is that the method cannot be applied to measurement on black toner where scattered light does not occur. It is also desired that toner amount measurement be conducted on black toner like color toner; the fact that the amount of the black toner cannot be measured by the method involves a problem.
The second problem is that it is difficult to apply the toner amount measurement method using scattered light on the type of photo sensitive body currently mainstream for the reason described later.
FIG. 6 is a drawing to show the structure of a surface of the type of photosensitive body currently mainstream.
The surface of the photosensitive body has a structure wherein an undercoat layer 1_2, a charge generation layer 1_3, a charge transport layer 1_4, and an overcoat layer 1_5 are laid up in order on an aluminum base material 1_1. In the currently mainstream image formation apparatus, to form an electrostatic latent image on the photosensitive body having such a surface structure, laser light is applied to the photosensitive body surface for generating charges in the charge generation layer 1_3 and the charges are held in the charge transport layer 1_4, whereby an electrostatic latent image is formed.
If the aluminum base material 113 1 has a smooth surface, the laser light incident through the photosensitive body surface and laser light reflected on the surface of the aluminum base material 1_1 interfere with each other and a desired electrostatic latent image cannot be provided. Thus, coarse surface working is conducted on the surface of the aluminum base material 1_1. If such a photosensitive body is used as a toner support and light L1 is made incident and scattered light L3 is received by the photosensor 6 as shown in FIG. 3, the photosensor outputs a voltage as described below:
FIG. 7 is a graph to show the relationship between the toner deposition amount and the outputs a voltage of the photosensor when the photosensitive body having the surface structure shown in FIG. 6 is used.
Like the graph of FIG. 4, the graph of FIG. 7 shows the amount of toner on the horizontal axis and the output voltage of the photosensor on the vertical axis, and the output voltage of the photosensor corresponds to the light amount of the scattered light caused by the toner.
When the toner deposition amount is small, the scattered light component from the base material surface of the photosensitive body is dominant and the scattered light has a high strength and the output voltage of the photosensor is high. As the toner deposition amount increases, the photosensitive body surface is covered with the toner and thus the scattered light from the base material surface decreases and the output voltage is lowered. When the toner deposition amount further increases, the scattered light component caused by the toner becomes dominant and as the toner deposition amount increases, the scattered light strength also increases and the output voltage is raised. Consequently, a curve 7xe2x80x2 in the graph meanders and it is difficult to measure the true value of the toner amount based on the curve 7xe2x80x2. Thus, it is difficult to apply the toner amount measurement method using scattered light to the currently mainstream photosensitive body. This is the second problem involved in the toner amount measurement method using scattered light.
The toner amount measurement method using specularly reflected light and the toner amount measurement method using scattered light involve their respective problems as described above. Thus, in the image formation apparatus in the related art, the fact is that the image formation condition is controlled based on the toner amount measurement result on the photosensitive body for a toner image having a reasonably small toner amount, the result of measuring representatively the toner amount on any other than the photosensitive body, such as a transfer belt, or the like. However, to form a high-quality image, it is desired that the toner amount should be measured on the photosensitive body for a toner image having a large toner amount and that the image formation condition should be controlled based on the measurement result.
A method of sucking toner on the photosensitive body and measuring the weight of the toner is available as the method of measuring the toner amount for a toner image having a large toner amount on the photosensitive body. That is, the image formation apparatus is shut down and the photosensitive body on which toner is deposited is removed before the toner is sucked and the weight of the toner is measured. However, a machine using such a measurement method is large and is hard to be housed in an image formation apparatus. Since such a method involves removing parts in measurement, a large number of steps are required for measurement execution and it is extremely difficult to measure the toner amount while the image formation apparatus is operated.
It is therefore an object of the invention to provide an image formation apparatus capable of measuring the toner amount on a photosensitive body during the operation for a toner image having a high toner amount and a toner amount measurement apparatus and a toner amount measurement method capable of measuring the toner amount on a photosensitive body for a toner image having a high toner amount.
To the end, according to one aspect of the invention, there is provided an image formation apparatus comprising:
a photosensitive body;
a first light application section for applying light to a surface of the photosensitive body for forming an electrostatic latent image;
a developing section for depositing toner on the electrostatic latent image formed by the first light application section for developing the electrostatic latent image; and
a transfer section for finally transferring onto paper a developed image into which the electrostatic latent image is developed by the developing section, thereby forming a toner image on the paper, wherein at least any one of the photosensitive body, the first light application section, the developing section, and the transfer section conforms to an image formation condition that can be controlled, characterized by:
a second light application section for applying light to the surface of the photosensitive body on which the toner is deposited;
a potential measurement section for measuring a surface potential of the photosensitive body to which the light is applied by the second light application section;
a toner amount derivation section for deriving the toner amount on the photosensitive body based on the surface potential measured by the potential measurement section; and
a condition control section for controlling the image formation condition in response to the toner amount derived by the toner amount derivation section.
To the end, according to another aspect of the invention, there is provided a toner amount measurement apparatus comprising:
a light application section for applying light to a surface of a photosensitive body supporting toner on the surface;
a potential measurement section for measuring a surface potential of the photosensitive body to which the light is applied by the light application section; and
a toner amount derivation section for deriving the toner amount on the photosensitive body based on the surface potential measured by the potential measurement section.
To the end, according to another aspect of the invention, there is provided a toner amount measurement method comprising:
a light application step of applying light to a surface of a photosensitive body supporting toner on the surface;
a potential measurement step of measuring a surface potential of the photosensitive body to which the light is applied at the light application step; and
a toner amount derivation step of deriving the toner amount on the photosensitive body based on the surface potential measured at the potential measurement step.